The invention relates to optical fiber lasers, and in particular continuous wave operation optical fiber lasers.
Many applications require coherent laser emission in wavelength regimes (e.g. ultraviolet, visible and mid-infrared) that are not readily accessible by conventional diode-pumped solid-state lasers and fiber lasers. The most popular way to reach these wavelength regimes is via nonlinear frequency conversion (e.g. second harmonic generation, sum-frequency generation, difference-frequency generation, optical parametric generation) of near-infrared laser emission from solid-state lasers and/or fiber lasers. The efficiency of each nonlinear frequency conversion process strongly depends on the optical power of the interacting beams [1]. Simple single-pass nonlinear frequency conversion in a suitable nonlinear crystal is a popular approach for high peak power pulsed laser sources, but is generally not very effective with lasers operating in continuous-wave (cw) mode as cw power levels are limited. One way to overcome this problem is to exploit the high intracavity powers that can be achieved in bulk solid-state lasers for efficient nonlinear frequency conversion. This approach is best known in the context of intracavity second harmonic generation and has become the standard method for producing multiwatt visible (green) output from neodymium-doped and ytterbium-doped solid-state lasers operating in the ˜1 μm wavelength regime. The success of this approach is largely due to the ability to construct solid-state laser resonators with very low round-trip cavity loss as this is a prerequisite for high intracavity power. As a result, intracavity frequency doubling of diode-pumped bulk solid-state lasers remains the most popular approach for generating multi-watt level, single spatial mode, continuous-wave laser radiation in the visible spectral region [2]. Unfortunately, power levels in conventional solid-state lasers are limited by thermal effects, which degrade efficiency and beam quality as pump power is increased. Indeed, thermal effects can be especially detrimental to the performance of intracavity frequency-doubled solid-state lasers by virtue of the increased cavity loss associated with thermally-induced phase distortion. As a consequence, this approach is generally limited to output power levels around a few tens-of-watts. A further drawback of solid-state laser gain media is that the emission bands tend to be quite narrow limiting the range of operating wavelengths.
In contrast, fiber lasers benefit from a geometry that offers a high degree of immunity from the effects of heat generation in the core. Waste heat generated by the laser pumping cycle is distributed over a long device length facilitating heat sinking and reducing the risk of thermally-induced damage. Moreover, the output beam quality is determined mainly by the waveguiding properties of the active-ion-doped core, which can be tailored to produce a single-spatial-mode output. As a consequence, fiber-based laser sources can be scaled to very high power levels (e.g. by using a cladding-pumped fiber architecture), whilst maintaining good beam quality and high efficiency. Indeed, recent advances in cladding-pumped fiber laser technology have been dramatic yielding multi-kilowatt, single-spatial-mode cw output in the ˜1 μm spectral region from ytterbium-doped fiber lasers [3]. A further attraction of fiber gain media is that the emission bands tend to be quite broad (as a consequence of the glass host) giving flexibility in operating wavelength. Thus fiber lasers also offer the prospect of high cw power in other wavelength regimes (e.g. UV, visible, mid-infrared).
Unfortunately, intracavity nonlinear frequency conversion schemes are not well suited for fiber lasers because of their relatively high resonator loss. As a result, the intracavity power attainable is generally not much higher than the output power that can be achieved with the optimum output coupling transmission, and hence there is only a small improvement in nonlinear frequency conversion efficiency compared to a simple single-pass conversion scheme [4]. One solution to this problem is to employ an external (to the laser) resonant enhancement cavity [5]. In this approach, the output power from the laser is enhanced via resonance in a low-loss external cavity, thereby avoiding the limitations associated with the high internal losses in the fiber source. This approach has been successfully applied to CW fiber-based sources for frequency doubling [6], but suffers from the drawback of added complexity since a single-frequency fiber master-oscillator power-amplifier (MOPA) is required, and precise control and active stabilization of the master-oscillator cavity length and/or resonant cavity length is needed to ensure that the resonance condition is maintained at all times. Furthermore, the output power available from the single-frequency MOPA, and hence the frequency doubled power is strongly limited by stimulated Brillouin scattering (SBS) in the amplifier fiber.